Single crystal superalloy vanes have demonstrated excellent turbine engine performance and durability benefits as compared with equiaxed polycrystalline turbine vanes. For a detailed discussion see “Allison Engine Testing CMSX-4® Single Crystal Turbine Blades & Vanes,” P. S. Burkholder et al., Allison Engine Co., K. Harris et al., Cannon-Muskegon Corp., 3rd Int. Charles Parsons Turbine Conf., Proc. Iom, Newcastle-upon-Tyne, United Kingdom 25–27 April 1995. The improved performance of the single crystal superalloy components is a result of superior thermal fatigue, low cycle fatigue, creep strength, oxidation and coating performance of single crystal superalloys and the absence of grain boundaries in the single crystal vane segments. Single crystal alloys also demonstrate a significant improvement in thin wall (cooled airfoil) creep properties as compared to polycrystalline superalloys. However, single crystal components require narrow limits on tolerance for grain defects such as low angle and high angle boundaries and solution heat treatment-induced recrystallized grains, which reduce casting yield, and as a result, increase manufacturing costs.
Directionally solidified castings of rhenium-containing columnar grain nickel-base superalloys have successfully been used to replace first generation (non-rhenium-containing) single crystal alloys at a cost savings due to higher casting yields. However, directionally solidified components are less advantageous than single crystal vanes due to grain boundaries in non-airfoil regions, particularly at the inner and outer shrouds of multiple airfoil segments exhibiting high, complex stress conditions. Multiple airfoil segments are of growing interest to turbine design engineers due to their potential for lower machining and fabrication costs and reduced hot gas leakage. Increased operating stress and turbine temperatures combined with the demand for reduced maintenance intervals has necessitated the enhanced properties and performance of single crystal rhenium-containing superalloy vane segments.
Thus, there is a recognized need for achieving the benefits of single crystal casting technology while also achieving increased tolerance for grain defects to improve casting yield and reduce component cost.